Gradient refractive index (GRIN) optical structures are composed of an optical material whose index of refraction, n, varies along a spatial gradient in the axial and/or radial directions of the lens. They have many useful applications such as making compact lenses with flat surfaces.
There are several known techniques for fabricating GRIN lenses. One approach is to press films of widely varying refractive indices together into a lens using a mold, e.g., as taught in U.S. Pat. No. 5,689,374. This process, however, is expensive to develop. A second approach for fabricating GRIN lenses is to infuse glass with ions at varying density. This approach has reached commercial production, but it is also expensive and effectively limited to small radially symmetric lenses by the depth to which ions will diffuse into glass. A third approach for fabricating GRIN lenses is to use 3D printing technology with inks composed of a polymer matrix doped with particles which change the index of refraction of the matrix. Each printed droplet has a distinct refractive index controlled by the concentration of dopants in the polymer material. This approach is described, for example, in R. Chartoff, B. McMorrow, P. Lucas, “Functionally Graded Polymer Matrix Nano-Composites by Solid Freeform Fabrication”, Solid Freeform Fabrication Symposium Proc., University of Texas at Austin, Austin, Tex., August, 2003, and in B. McMorrow, R. Chartoff, P. Lucas and W. Richardson, ‘Polymer Matrix Nanocomposites by Inkjet Printing’, Proc. of the Solid Freeform Fabrication Symposium, Austin, Tex., August, 2005.
Although using 3D printing has the potential to provide an efficient and inexpensive means of fabricating GRIN lenses, a number of unsolved problems have prevented or significantly limited its practical realization. One of the most significant problems is that the ink compounds need to simultaneously have all the desired properties for high quality GRIN lenses while at the same time need to have properties suitable for 3D printing using inkjet technology. In particular, it is important that the doping of the host matrix creates a substantial change in the index of refraction of the host matrix, so that the GRIN lens can efficiently provide significant optical power. It is also important that the material, both when doped and undoped, be substantially transparent at wavelengths of interest (e.g., visible spectrum) so that light is transmitted through the lens rather than absorbed. At the same time, in order to be suitable for the 3D printing process, the ink material must have a low viscosity both with and without doping, and be curable by a process that does not create uncontrollable distortion of the printed lens. Despite the desirability for an ink satisfying all of these criteria, researchers have yet to understand what physical characteristics of matrix and dopant materials are sufficient to produce inks satisfying all these properties, or to discover any specific ink compounds that simultaneously possess all these properties. As a result, the realization of 3D printing of high quality GRIN lenses remains elusive.